1. Field of the Invention
The present invention relates to a method of preparing a cylindrical aluminum substrate for electrophotographic photoreceptors. The cylindrical substrates, "drums", are used in photocopiers, laser printers and similar devices.
2. Discussion of the Background
Several methods of preparing a cylindrical aluminum substrate ("aluminum" includes not only aluminum metal of a single substance but also aluminum alloys) which is used as a substrate of an electrophotographic photoreceptor, are known including: (1) preparing a bottomed cylinder in which aluminum is formed into a cup-shaped article by deep-drawing and then the wall of the cup is expanded by ironing to form a bottomed aluminum cylinder (Drawing and Ironing method); (2) preparing a cylinder in which aluminum is formed into a cup-shaped article by impact extrusion and then the wall of the cup is expanded by ironing to form an aluminum cylinder (Impact and Ironing method); (3) preparing a thin-walled cylinder in which an aluminum cylinder, obtained by extrusion, is expanded by ironing to form a thin-walled aluminum cylinder (Extrusion and Ironing (EI) method); and (4) preparing a thin-walled cylinder in which an aluminum cylinder, obtained by extrusion, is subjected to deep-drawing to form a thin-walled aluminum cylinder (Extrusion and Drawing (ED) method); as well as methods of cutting and machining the cylindrical article prepared by one of these methods. In the EI method and ED method as well as those followed by a cutting step, extrusion is effected as the inter-working step.
Two means of preparing a hollow pipe by extrusion working are known, one is a mandrel system process and the other is a porthole system process. In the mandrel system process a mandrel is fixed at the top of the stem of the extruder and is used as a core and a hollow pipe is prepared by extrusion. However, the process has several drawbacks in that the wall of the hollow pipe is often uneven and formation of a thin-walled pipe is difficult. Therefore, the porthole system process is employed most often in producing a cylindrical aluminum substrate for electrophotographic photoreceptors. In the porthole system, the metal is extruded into a multi-part die, usually having two parts, and the parts are welded in the mold to form a hollow pipe therein. The system is superior to the mandrel system process since it forms even walls and is capable of forming thin-walled hollow pipe. However, the system has one drawback, the hollow pipe formed has a weld line caused by the welding.
The weld line would not cause any particular problem when the hollow pipe formed is used in general structures, since a favorable metal-constitutional conjugation may be attained in the welded part of the pipe provided that the extrusion condition is suitably adjusted. However, when a cylindrical aluminum substrate is formed by extrusion in the port-hole system process and is coated with a photo-conductive layer to form an electrophotographic photoreceptor, streak defects appear in the position corresponding to the weld line in the substrate. In severe cases the defects may be detected with the naked eye on the photo-conductive layer as formed on the electrophoto-graphic photoreceptor. However, even when such defects can not be detected with the naked eye, they cause image defects running along the direction of the axis of the cylindrical substrate on the duplicated images formed by the electro-photographic photoreceptor, i.e. the image contains streaks.
Thus, an object of the present invention is to provide a method of preparing a cylindrical aluminum substrate for an electrophotographic photoreceptor, which is free from the defects in the position corresponding to the weld line formed during extrusion of the cylindrical aluminum substrate and is also free from streak defects in the duplicated images formed by the cylindrical aluminum substrate electrophotographic photoreceptor, the drum, prepared from the cylindrical aluminum substrate.
Another object of the present invention is to provide a method of preparing an electrophotographic photoreceptor drum which has very few defects and its use results in images having very few defects. Still another object of the present invention is to provide an excellent and defect-free electrophotographic drum.
The present inventors have found that the above-mentioned defects are derived from the oxides which are on the surface of the mold and are incorporated onto the weld line from the surface of the mold during extrusion. Specifically, since the oxides incorporated onto the weld line from the surface of the mold are extremely hard and brittle, as compared with the aluminum matrix, they are often broken by the drawing, ironing or cutting work applied to the surface of the substrate after the extrusion and form pores, or the oxides remain unprocessed and form projections. If a photo-sensitive layer is coated on the substrate having such pores or projections on the weld line thereof, thin and uneven portions of the photo-sensitive film are formed around the pores or projections. It has been found that such thin and uneven portions cause streak defects on the electrophotographic photoreceptor and also cause streak-like patterns on the images formed by the defective electrophotographic photoreceptor.
Accordingly, to overcome such defects, it is recommended to employ a means which avoids oxidizing the surface of the extrusion mold used for extrusion of the cylindrical aluminum substrate.
In general, an extrusion mold is made of an alloy tool steel of JIS (Japanese Industrial Standard) type. In the extrusion of aluminum, in general, the cavity of the extrusion mold after completion of one extrusion step contains an aluminum residue. Therefore, it is a general practice to remove the aluminum residue from the cavity of the extrusion mold by the use of an alkali substance such as sodium hydroxide or the like, in the interval after completion of the extrusion step and before the next extrusion step. Because of the alkali treatment, the surface of the extrusion mold made of an alloy tool steel, after the aluminum residue has been removed, is coated with an oxide film.
Further, in the next step, the extrusion mold is heat-treated, generally at a temperature of from 450.degree. to 550.degree. C. for several hours. The heat-treatment step, further oxidizes the surface of the extrusion mold made of an alloy tool steel resulting in an oxide film coating.